1. Field of the Invention
The present invention relates to a method and device to correct the variation or drift of the phase difference xcex94"PHgr" between two signals generated by a frequency signal Fe=2"PHgr"xcexa9, a drift in the value of Fe prompting the drift in the value of the phase difference.
The invention can be applied especially to the correction of the drift in the value of the QDM (the code commonly used in navigation to designate the angle xcex8 between a moving body and the magnetic north) in order to meet certain standards such as EUROCAE ED-22B, RTCA, ARINC MARK 2 AIRBORNE VOR RECEIVER 711-9. This drift results from a variation in the rotation frequency.
The invention can be applied for example but not exclusively, to air, sea or river navigation.
2. Description of the Prior Art
FIG. 1 shows the principle of operation of a system used, on board a moving body or aircraft, for example an aircraft, to determine the direction of this moving body at a given place, as seen from a radio beacon. A continuous wave is sent in a cardioidal radiation pattern rotating at 30 rpm. At the same time, an omnidirectional antenna sends a reference signal at 30 Hz with frequency modulation. The on-board receiver detects the amplitude modulation at 30 Hz caused by the rotation of the pattern and the reference signal. The azimuth of the receiver seen from the station or VOR beacon is equal to the phase shift between the two signals E1 and E2. E1 is the modulation due to the rotation of the pattern and E2 is the reference transmitted.
A variation in the 30 Hz frequency prompts a linear drift in the phase difference and the beat introduced into the signal as can be seen in FIGS. 2 and 3.
FIG. 4 is an exemplary processing chain according to the prior art.
A standard digital VOR receiver technique consists in applying a separator filter to process two frequency bands, the AM modulated low-frequency band and the FM modulated high-frequency band, applying a frequency discriminator to recover the FM modulating frequency and comparing the phases of the two signals.
The composite signal VOR, after reception by a receiver 10, is separated by means of a separator 11 into an AM modulated low-frequency signal (LF) and an FM modulated high-frequency signal (HF).
The low frequency signal (LF) is sent to a linear filter 12 and then sampled 13 in order to determine its phase "PHgr"1 by means of a Fourier transform.
The high frequency signal (HF) is sent first of all to a non-linear filter 14 and then sampled 16 in order to determine its phase "PHgr"2 in implementing a Fourier transform. The non-linear signal has a frequency discriminator 15 which, in particular, has the function of recovering the FM modulating frequency.
Using the two phases "PHgr"1 and "PHgr"2, the value of the phase shift xcex94"PHgr" corresponding to the QDM (plus or minus the parameter of the phase-shift terms introduced by the filters) is determined for example by a computer programmed accordingly.
During the signal processing method, the operation of sampling the (LF) and (HF) signals by Fourier transform may generate spurious lines when the period of the analyzed signal is not a multiple of the temporal window of analysis.
To overcome this problem, there are known ways of using standard windows such as a generalized Hamming window to reduce the side lobes of the spectrum while widening the major lobe or the Kaiser lobe associating an infinite spectrum with a time signal with finite support. These two methods however have limited performance characteristics and disturb the signal-to-noise ratio for equal integration time.
It is also possible to use a DFT type filter bank method. In this case, the computation power needed is very great and leads to investment costs that are incompatible with certain applications.
FIG. 3 gives a view, in a referential system where the X-axis is the time axis and the Y-axis represents the drift in the QDM, of:
The curve (I) which corresponds to the value QDM(Fe) for the reference frequency Fe or the corresponding period xcexa9.
The curve (II) which corresponds to the drift in the QDM and the curve (III) which corresponds to a beat prompted by the drift in the value of the reference frequency Fe.
Hereinafter in the description, the term QDM designates the angle taken with respect to the magnetic north, also known as the bearing.
The invention consists especially of the use of a relationship linking the drift in the QDM with a variation in the reference frequency and a frequency estimator carefully positioned in the processing chain to correct this drift.
It can also use a trapezoidal sampling window, for example to carry out the Fourier transform.
The invention relates to a method to correct the drift in the phase shift (xcex94"PHgr", xcex8) between two signals (S1) and (S2), said signals being generated by a signal having a reference frequency Fe or a corresponding period xcexa9, the drift in the phase shift resulting from a drift in frequency Fe or in period depending on a given relationship R((F,xcexa9), (xcex8, xcex94"PHgr")).
The invention comprises at least the following steps:
determining the value of the phase shift (xcex94"PHgr", xcex8) from the phase "PHgr"1 of the signal S1 and the phase "PHgr"2 of the signal S2,
estimating Fest the value of frequency Fe, or the corresponding period xcexa9,
bringing this value (xcex94"PHgr", xcex8) into an interval of given values [QDMmin, QDMmax] containing the value QDM(Fe) obtained for the reference value Fe taking account of the values xcex94"PHgr" and Fest and of the given relationship R((F,xcexa9), (xcex8,xcex94"PHgr")).
According to one embodiment, the method comprises, for example, a step of processing by Fourier transform using a sampling window whose shape is substantially trapezoidal.
The invention also relates to a method to correct a drift in the phase shift (xcex94"PHgr", xcex8) between two signals, S1 having a phase "PHgr"1 and S2 having a phase "PHgr"2, the two signals being generated by a signal with a frequency Fe or with the corresponding period, the drift in the phase shift resulting from the drift in Fe or the drift in the period according to a given relationship R((F,xcexa9), (xcex8, xcex94"PHgr")). The method comprises at least one step to determine at least one of the phases ("PHgr"1, "PHgr"2) by Fourier transform using a substantially trapezoidal sampling window.
The invention also relates to a device for the correction of the drift in the phase shift (xcex94"PHgr",xcex8) between two signals (S1) and (S2), said signals being generated by a signal with a reference frequency Fe, the drift being linked to the drift in frequency Fe according to a given relationship R((F,xcexa9), (xcex8, xcex94"PHgr")). The device comprises at least:
a separator of the signals S1 and S2, means to determine the value of the phase "PHgr"1 and "PHgr"2 for each of the signals and means to determine the value of the phase shift xcex94"PHgr",
an estimator of the frequency Fe or of the corresponding period positioned after the separator,
a device comprising, in memory, the relationship R((F,xcexa9), (xcex8, xcex94"PHgr")) receiving the values xcex94"PHgr" and the estimated frequency or the estimated corresponding period, adapted to bringing the value of xcex94"PHgr" to a value within an interval of given values [QDMmin, QDMmax] containing the value QDM(Fe) obtained for the reference value Fe (or period).
According to one embodiment, the device comprises for example means to process the signals (S1) and/or (S2) by Fourier transform, the sampling window having a substantially trapezoidal shape.
The estimator to estimate the frequency value may be of the Prony""s estimator type.
The invention also relates to a VOR receiver comprising a device for the correction of a drift in the phase shift (xcex94"PHgr", xcex8) between two signals (S1) and (S2), said signals being generated by a signal having a reference frequency Fe, the drift being linked to the drift in frequency Fe or to the drift in period according to a given relationship R((F,xcexa9), (xcex8, xcex94"PHgr")), comprising at least:
a separator of the signals S1 and S2, means to determine the value of the phase "PHgr"1 and "PHgr"2 for each of the signals and means to determine the phase shift value xcex94"PHgr",
an estimator of the frequency Fe or of the corresponding period positioned after the separator,
a device comprising, in memory, the relationship R((F,xcexa9), (xcex8, xcex94"PHgr")) receiving the values xcex94"PHgr" and the estimated frequency or the estimated corresponding period, and adapted to bringing the value of xcex94"PHgr" to a value within an interval of given values [QDMmin, QDMmax] containing the value QDM(Fe) obtained for the reference value Fe (or period).
The methods and devices according to the invention can be applied for example to reducing the variation of the QDM of a navigation system or radio navigation system.
The method and device according to the invention can be used especially to achieve compliance with stringent standards, such as RTCA and EUROCAE standards, or to approach them as closely as possible. The invention also improves the limit of detection as compared with the prior art devices.